The use of polymers as viscosity modifiers or dispersant viscosity modifiers (otherwise known as VM's) in an oil of lubricating viscosity is well known. A good VM will react to ambient temperatures to provide the desired level of viscosity modification. Poor performing VM's will cause, for example, undesirable increases in viscosity at low temperature, and/or undesirable effects upon degradation. For example, at high temperatures, such as those found in a piston cylinder, inefficient degradation of the viscosity modifier will result in burning of the degradants and carbon buildup in the cylinder. Away from high temperatures, inefficient degradation can lead to oxidation of the viscosity modifier, resulting in an undesirable increase in viscosity.
Limited cleanliness is often observed in engine piston deposits particularly for olefin copolymer viscosity modifiers, such as ethylene-propylene copolymers (OCP's). Although OCP's provide good low temperature performance, OCP's have a tendency to oxidize and also inefficiently degrade at high temperatures.
In contrast to OCP's, poly(vinyl aromatic) polymers (PVA polymers), such as poly(styrene), degrade much more efficiently, but are oil insoluble. Oil solubility can be imparted to PVA polymers, such as styrene, by copolymerization with a diene, such as butadiene. In fact, styrene butadiene copolymers have been investigated as viscosity modifiers or dispersant viscosity modifiers that have shown marked improvement in piston deposits over mainline ethylene-propylene copolymers. For example, U.S. Publication No. 2010/0152082 discloses styrene-butadiene block copolymers created by anionic polymerization. However, dienes do not degrade efficiently and need to be functionalized, for example, by the addition of pendant carbonyl groups.
It is known that poly(isobutylene) (PIB) will not oxidize and will efficiently degrade at high temperatures. However, PIB's do not exhibit good thickening performance and exhibit poor low temperature performance.
A copolymer capable of being used as a VM that can provide at least one of acceptable low temperature performance and/or cleanliness and that can be produced simply is desired.